If the motor runs up to 2500 and then sounds like its hitting a revs limiter then you may have a plugged catalytic convertor. an easy test to see if the convertor is working all right is to get a inf red thermometer and aim it at the pipe just in front of the cat and then on the pipe just behind the cat . The motor needs to be hot when you do this and rev the motor at 2000rpm . The temp should be 100+ degrees hotter on the backside of the cat . If the temp is higher before the cat than after the cat then the cats not working . If the temp is the same at both ends its not working . Hope this helps